PARIS (REUTERS ) - French far-left candidate Jean-Luc Melenchon is seen beating conservative Francois Fillon to third place in the first round of the April-May presidential election, according to a poll by Kantar Sofres for Figaro-LCI-RTL published on Sunday (April 9).

It was the first poll to put Melenchon ahead of Fillon in the election cycle. Melenchon had gained six percentage points since mid-March to 18 per cent, newspaper Le Figaro said, while Fillon was stable at 17 percent.

Melenchon drew tens of thousands of supporters to a mass open-air rally on Sunday, underlining his surging popularity just two weeks from the unpredictable election.

Polls show far-left Melenchon closing in on the frontrunners, 39-year-old centrist Emmanuel Macron and far-right leader Marine Le Pen ahead of the April 23 vote, adding new drama to a rollercoaster campaign.

Macron and Le Pen, each with 24 per cent, were down 2 percentage points each since mid-March. Socialist Party candidate Benoit Hamon was down 2 per cent to 9 per cent.

After strong performances in two televised debates, several new surveys this weekend showed Melenchon climbing to third position, with 18-19 per cent of voters saying they would vote for him.

Speaking in southern Marseille, he said voters had a choice other than the extreme-right "condemning our great multi-coloured people to hate itself" and fans of the free-market that "transforms suffering, misery and abandonment into gold and money".

Left-leaning news magazine L'Obs commented on Sunday that "the sudden emergence of Jean-Luc Melenchon among the four candidates with around 20 per cent has shattered all the predictions, (and) is sowing doubt among the favourites".

Analysts say forecasting the French two-stage election is even more difficult than usual, with an unusually high number of voters saying they do not plan to cast their ballots, or have not made up their minds.

Elsewhere on Sunday, scandal-hit rightwinger Fillon held one of his biggest rallies so far, gathering thousands of flag-waving supporters at a conference hall in southern Paris.

The 61-year-old ex-prime minister is desperate to pick up momentum heading into the closing fortnight after a campaign dominated by allegations he paid his wife hundreds of thousands of euros for a fake job in parliament.

"If he doesn't rise a few points (in the polls) this week, it's over," one former minister and Fillon ally admitted on condition of anonymity.

During the meeting, Fillon kept up his attacks on election favourite Macron, painting him as the continuation of unpopular Socialist President Francois Hollande, whom Macron served for four years as advisor then as economy minister.

"France would be the big loser: another five years of half-measures, another five years of missed opportunities," he said to cheers.

While Melenchon and Fillon gathered supporters, Macron and Le Pen were happy to spend time giving interviews.

Macron detailed what would be his priorities for his first few months in office to the Journal du Dimanche newspaper, saying one of his first measures would be to pass a law setting new ethical standards for parliament.

This would be followed by other legislation to cut the number of MPs by a third and to free up the labour market.

Asked about a slight fall in support according to recent surveys, he replied: "They show exactly what I feel: that nothing is decided yet. We are entering a crucial phase."

In a sign that his team are growing anxious about the impact of Melenchon, particularly among the young, supporters spread a video online set to techno music warning about the leftwinger's huge tax-and-spend plan.

Melenchon's radical programme includes a new 100-billion-euro ($149 billion) stimulus plan and a reduction in the working week to 32 hours, as well as proposals to overhaul the European Union and pull France out of NATO.

Le Pen meanwhile and her closest allies hit the airwaves selling their vision of a nationalist France, unburdened by the European Union and the euro currency and tougher on crime and Islamists.

Her telegenic niece Marion Marechal-Le Pen made a rare television appearance to deny reports of differences and tensions in the family, which led to speculation this week she might quit.

"I'm 100 per cent behind Marine Le Pen," she said on BFM television, adding that she planned to stand in parliamentary elections in June.

Two new polls published this weekend confirmed shifting momentum seen earlier this week following a second and final televised debate between the 11 candidates vying to be France's next leader.

Le Pen and Macron - dubbed the "fragile favourite" by the Journal de Dimanche - are neck-and neck but both have lost ground slightly and would win 23-24 per cent if the vote were held today.

This would mean they both qualified for the second round run-off scheduled on May 7, which Macron is seen winning comfortably.